Illumination optical system, alignment apparatus, and projection exposure apparatus using the same

ABSTRACT

An illumination optical system and an alignment apparatus suitable for reticle alignment in the projection exposure apparatus. There is provided an illumination optical system having parallel beam supply means for supplying a parallel beam and a light guide for guiding the parallel beam from the parallel beam supply means to a target illumination object, comprising diffusion means, arranged between the parallel beam supply means and the light guide, for diffusing the parallel beam, wherein an incident end face of the light guide is arranged to be inclined by a predetermined angle with respect to a plane perpendicular to a direction of incidence of the parallel beam onto the diffusion means.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an illumination optical system, andmore particularly, an illumination optical system used for a reticlealignment optical system of a projection exposure apparatus, and analignment apparatus, and more particularly, an alignment apparatussuitable for reticle alignment in the projection exposure apparatus.

2. Related Background Art

In the alignment optical system of a projection exposure apparatus, whenlight supply itself must have some degree of freedom to illuminate amoving target object, e.g., when a transparent alignment mark providedon an index plate on a movable stage is to be illuminated to cause lightto emerge therefrom, an illumination optical system having a light guideconsisting of an optical fiber is used as an illumination optical systemfor illuminating the alignment mark. FIG. 1 is a view showing such aconventional illumination optical system using an optical fiber.

Referring to FIG. 1, a light beam radiated from a mercury lamp 300serving as a light source is focused upon reflection by an ellipticalmirror 301 and incident on an optical fiber 303 through a lens system302. At this time, a light source image having a sufficient numericalaperture and diameter is formed at the incident end of the optical fiber303. The light beam emerging from the optical fiber 303 forms anillumination field having an appropriate diameter and illuminationnumerical aperture through a condenser lens 304.

On the other hand, in manufacturing a semiconductor element or a liquidcrystal display element, a projection exposure apparatus is used inwhich a reticle (or a photomask or the like) is illuminated withexposure light, and the pattern of the reticle is focused and projectedonto a wafer (or a glass plate or the like) coated with a photoresistthrough a projection optical system. Conventionally, continuouslyemitted light such as a beam from a mercury lamp (e.g., the g-ray or thei-ray) is used as the exposure light. In such a projection exposureapparatus, the reticle and wafer must be accurately aligned with eachother in the exposure operation. For this purpose, first of all, thereticle is aligned with respect to a reticle stage by using ameasurement result from a reticle alignment system.

Upon completion of reticle alignment, the position of the reticle ismeasured with reference to a wafer stage on which the wafer is mounted.The wafer is aligned on the basis of the measurement result. To measurethe positional relationship of the reticle with respect to the waferstage, a reticle position detection system of a so-called stage lightemission type is used in some cases.

SUMMARY OF THE INVENTION

According to the present invention, there is provided an illuminationoptical system having parallel beam supply means for supplying aparallel beam and a light guide for guiding the parallel beam from theparallel beam supply means to a target illumination object, comprisingdiffusion means, arranged between the parallel beam supply means and thelight guide, for diffusing the parallel beam, wherein an incident endface of the light guide is arranged to be inclined by a predeterminedangle with respect to a plane perpendicular to a direction of incidenceof the parallel beam onto the diffusion means.

Relation 4°≦θ≦10° is preferably satisfied, where θ is an inclinationangle of the incident end face of the light guide with respect to theplane perpendicular to the direction of incidence of the parallel beamonto the diffusion means.

According to the present invention, an alignment apparatus is providedto a projection exposure apparatus having an illumination optical systemfor illuminating a mask on which a target transferred pattern is formedwith pulsed exposure light, a projection optical system for projectingan image of the pattern of the mask onto a photosensitive substrate withthe exposure light, and a substrate stage for holding the substrate andpositioning the substrate on a plane perpendicular to an optical axis ofthe projection optical system, and detects a position of the mask byusing an alignment mark formed on the mask and a reference mark arrangedon the substrate stage.

The alignment apparatus according to the present invention comprises afirst alignment illumination system for illuminating the alignment markon the mask with continuously emitted continuous illumination light in awavelength range different from that of the exposure light, a secondalignment illumination system for illuminating the alignment mark andthe reference mark on the substrate stage with pulse illumination lightobtained upon branching the pulsed exposure light, a first objectiveoptical system for focusing the continuous illumination light from thealignment mark, the pulse illumination light from the alignment mark,and the pulse illumination light from the reference mark through theprojection optical system, a wavelength selection optical system fordividing a light beam focused by the first objective optical system intoa light beam by the continuous illumination light and a light beam bythe pulse illumination light, a second objective optical system forforming an image of the alignment mark from the light beam by thecontinuous illumination light divided by the wavelength selectionoptical system, image position detection means, having photoelectricdetection means, for relatively vibrating the photoelectric detectionmeans and the image of the alignment mark which is obtained by thecontinuous illumination light, thereby detecting a position of the imageof the alignment mark.

In addition, the alignment apparatus according to the present inventioncomprises a third objective optical system for forming the images of thealignment mark and the reference mark from the light beam of the pulseillumination light divided by the wavelength selection optical system,and image pickup means for picking up the images of the alignment markand the reference mark which are obtained from the pulse illuminationlight, wherein a positional relationship of the mask with respect to theimage position detection means is detected on the basis of a detectionresult from the image position detection means, and a positional shiftbetween the alignment mark and the reference mark on the basis of adetection result from the image pickup means.

In this case, it is preferable that the first objective optical systembe arranged to be movable in correspondence with the position of thealignment mark on the mask, and the apparatus further comprise acorrection optical system, arranged between the wavelength selectionoptical system and the second objective optical system, or between thewavelength selection optical system and the third objective opticalsystem, for converting the light beam from the first objective opticalsystem into a parallel beam in a manner interlocked with the firstobjective optical system.

When the first alignment illumination system performs incident-lightillumination of the alignment mark from an upper side of the mask, theapparatus preferably comprises a movable mirror, arranged to be freelyinserted between the mask and the projection optical system, forreflecting the continuous illumination light transmitted through themask toward the alignment mark.

In this case, the apparatus preferably comprises a field stop arrangedon a plane conjugate with an arrangement plane of the movable mirror inthe first alignment illumination system.

According to the alignment apparatus of the present invention, thealignment mark on the mask and the reference mark on the substrate stageside are illuminated by the second alignment illumination system usingthe pulse illumination light obtained upon branching the exposure light,and the images of the two marks are picked up by the image pickup means.Therefore, the positional relationship of the mask with respect to thesubstrate stage is measured on the basis of the obtained images. At thistime, since the pulse illumination light has the same wavelength as thatof the exposure light, the projection optical system has no chromaticaberration, and the two marks are sharply focused on the image pickupsurface of the image pickup means.

In addition, the image position detection means such as a photoelectricmicroscope for relatively vibrating the image of the alignment mark andthe photoelectric detection means to detect the position of the image ofthe alignment mark uses the continuous illumination light. For thisreason, the position of the mask with respect to the image positiondetection means is accurately detected.

When the first objective optical system is arranged to be movable incorrespondence with the position of the alignment mark on the mask, andan alignment mark at the different position is to be detected, the firstobjective optical system must be accordingly moved along the mask.However, the pulsed exposure light is generally in a far-ultravioletrange, and the usable glass material is limited. For this reason, it isdifficult to convert the light beam from the first objective opticalsystem into a parallel beam commonly for the pulse illumination lightand the continuous illumination light. When a parallel system is formedon the pulse illumination light side, the correction optical system isarranged between the wavelength selection optical system and the secondobjective optical system. This correction optical system converts thelight beam from the first objective optical system in a mannerinterlocked with the first objective optical system. With thisarrangement, even when the first objective optical system moves on themask, position detection can be accurately performed.

Furthermore, when the first alignment optical system performs incidentillumination of the alignment mark from the upper side of the mask, themovable mirror is inserted between the alignment mark on the mask andthe projection optical system. In this case, transmission illuminationof the alignment mark is performed with the light reflected by themovable mirror. Therefore, even when the alignment mark has a lowreflectance, position detection can be accurately performed.

At this time, when the field stop is arranged on the plane conjugatewith the arrangement plane of the movable mirror in the first alignmentillumination system, the same illuminance distribution can be obtainedat the alignment mark in either the presence or absence of the movablemirror.

The present invention will be more fully understood from the detaileddescription given hereinbelow and the accompanying drawings, which aregiven by way of illustration only and are not to be considered aslimiting the present invention.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will beapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view schematically showing a conventional alignmentillumination optical system;

FIG. 2 is a view schematically showing the illumination optical systemin FIG. 1 which uses an excimer laser source;

FIG. 3 is a view schematically showing the illumination optical systemusing a fly-eye lens;

FIG. 4 is a view schematically showing the illumination optical systemusing a diffusion plate;

FIG. 5 is a view of an optical path, which shows the opticalcharacteristics of a light guide consisting of a bundle of alignedoptical fibers;

FIG. 6 is a graph showing the angular characteristics of luminance oflight emerging from the light guide shown in FIG. 5;

FIG. 7 is a view of an optical path, which shows the opticalcharacteristics of a random light guide;

FIG. 8 is a graph showing the angular characteristics of luminance oflight emerging from the light guide shown in FIG. 7;

FIG. 9 is a view of an optical path, which shows the opticalcharacteristics of the random light guide having an incident surface asa perfect diffusion surface;

FIG. 10 is a graph showing the angular characteristics of luminance oflight emerging from the light guide shown in FIG. 9;

FIG. 11 is a graph showing the angular characteristics of luminance whena parallel beam is spread using a diffusion means;

FIG. 12 is a graph showing the angular characteristics of luminance ofthe incident light when the light beam spread by the diffusion means isobliquely incident on the light guide;

FIG. 13 is a graph showing the angular characteristics of luminance ofthe exit light when the light beam spread by the diffusion means isobliquely incident on the light guide;

FIG. 14 is a view schematically showing the function of the illuminationoptical system according to the present invention;

FIG. 15 is a graph showing the angular characteristics of luminance ofthe exit light from the light guide when an inclination angle θ issmaller than 4°;

FIG. 16 is a graph showing the angular characteristics of luminance ofthe exit light from the light guide when the inclination angle θ is 12°;

FIG. 17 is a view schematically showing a projection exposure apparatusby an ISS (image slit sensor) using the illumination optical systemaccording to the first embodiment of the present invention as analignment optical system;

FIG. 18 is a graph showing a matched state between a mark SM on theindex plate and a reticle mark RM in the projection exposure apparatusshown in FIG. 17;

FIG. 19 is a view showing an example of the reticle mark RM;

FIG. 20 is a view schematically showing a projection exposure apparatususing an illumination optical system according to the second embodimentof the present invention for an alignment optical system to performreticle alignment;

FIG. 21 is a view showing a transparent cross reticle mark RM used inthe projection exposure apparatus shown in FIG. 20;

FIG. 22 is a view showing an example of the reticle mark RM;

FIG. 23 is a view schematically showing a modification of theillumination optical system according to the first or second embodiment;

FIG. 24 is a view schematically showing another modification of theillumination optical system according to the first or second embodiment;

FIG. 25 is a view schematically showing an illumination optical systemapplied with part of the illumination optical system of the presentinvention;

FIG. 26 is a view showing the main part of a general projection exposureapparatus;

FIG. 27 is a plan view showing a reference mark provided on a stagesubstrate of the projection exposure apparatus shown in FIG. 26;

FIG. 28 is a plan view showing a reticle alignment system of theprojection exposure apparatus shown in FIG. 26;

FIG. 29 is a plan view showing the reticle alignment system of theprojection exposure apparatus to which an alignment apparatus accordingto an embodiment of the present invention is applied;

FIG. 30 is a view showing the entire projection exposure apparatus shownin FIG. 29;

FIG. 31 is a view for explaining a change in illuminance distribution bycontinuous illumination light;

FIG. 32 is a view for explaining transmission illumination of a reticleby a movable mirror;

FIG. 33 is a perspective view showing a relationship between the reticlemark as a mirror image and an actual reticle mark in FIG. 32; and

FIG. 34 is a view showing a random light guide and a diffusion platewhich are applied to the projection exposure apparatus shown in FIG. 30.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Currently, an excimer laser is used as a light source for the alignmentapparatus of a projection exposure apparatus. On the other hand, anoptical fiber compatible with the excimer laser in durability, such as aquartz fiber has been developed. Therefore, an alignment illuminationoptical system constituted by an optical fiber optical system using anexcimer laser as a light source and having some degree of freedom can beconsidered.

However, an excimer laser beam is a parallel beam with a highdirectivity. When this beam is used as illumination light, theillumination numerical aperture is almost zero. When the optical fiberoptical system of the prior art, as shown in FIG. 1, is used, not only adesired illuminance distribution cannot be obtained with a laser beamthrough a lens system 312, an optical fiber 313, and a condenser lens314, but also an illumination field itself cannot be formed. Therefore,the conventional system cannot be used as the illumination opticalsystem of an alignment apparatus.

To eliminate illuminance variations of the laser and ensure theillumination numerical aperture in the illumination optical system ofthis type, a method can be applied to the illumination optical system,in which the laser beam is caused to pass through a fly-eye lens 401 toform a secondary source, and Kohler illumination is performed with thelight beam from this secondary source through a condenser lens 402,thereby forming an illumination field, as shown in FIG. 3.Alternatively, a method can be applied in which the diffusion functionof a diffusion plate 411 is used to form the secondary source of a laserbeam on the exit surface, and Ktohler illumination is performed with thelight beam from this secondary source through a condenser lens 412,thereby forming an illumination field, as shown in FIG. 4.

In the system using the fly-eye lens 401 in FIG. 3, however, if thenumber of lenses of the fly-eye lens 401 is not sufficiently large(four-lens structure in FIG. 3), illuminance variations of the incidentlaser beam cannot be sufficiently corrected. On the other hand, anincrease in number of lenses is disadvantageous for the cost. Even alaser which is not so good in coherency, such as an excimer laserproduces interference fringes on the illumination field because oftwo-beam interference between adjacent lens elements, so uniformillumination cannot be obtained.

On the other hand, a system using the diffusion plate 411 in FIG. 4exhibits an illuminance variation distribution having a shape like thatof a regular distribution on the illumination field because of theangular characteristics after the light beam passes through thediffusion plate. For this reason, with the normal condenser lens 412,uniform illuminance distribution cannot be obtained, and only a limitedarea at the center of the illumination field, which allows to neglectilluminance variations, can be used, resulting in a degradation inillumination efficiency. In addition, even a laser which is not so goodin coherency, such as an excimer laser produces speckles on theillumination field to result in random illuminance variations.

If such an illumination optical system is used for an alignmentapparatus, alignment precision is degraded.

In the illumination optical system according to the present invention,when a parallel beam supplied from a parallel beam supply means is to beguided to a target illumination object side through a light guide, theparallel beam is diffused by a diffusion means arranged between theparallel beam supply means and the incident end of the light guide. Atthe same time, the incident end face of the light guide is arranged tobe inclined by a predetermined angle with respect to a planeperpendicular to the direction of incidence of the parallel beam ontothe diffusion means.

The light guide used here is a light guide constituted by a bundle ofthin glass fibers, in which the position of a certain single fiber onthe incident side section is arranged at random with respect to its exitside section (to be referred to as a random light guide hereinafter). Aproduct available from Hoya-Schott Kabushiki Kaisha can be used as therandom light guide. The optical nature of a random light guide excellentin randomness will be described below.

As shown in FIG. 5, a light guide 200 constituted by a bundle ofperfectly aligned optical fibers has an incident angle θ₁ almost equalto an exit angle θ₂. The section of the light beam is spread to form adoughnut-like shape because the light beam emerges in random directions.The distribution of light amount on the incident end face is alsomaintained on the exit end face (FIG. 6). However, since only the coreportions of the bundled optical fibers transmit a light beam, the lightamount in gaps between the optical fibers or in the clad portionsbecomes zero.

To the contrary, in a random light guide, the optical fibers are twistedinside at random, as shown in FIG. 7. For this reason, the exit angle ismaintained to spread with respect to the incident angle. This means thatthe random light guide has an angular diffusion effect. Since the lightbeam emerges in random directions, the section of the light beam has adoughnut-like shape, and the distribution of light amount becomessymmetrical about its center. The distribution of light amount can beregarded to be almost uniform on the exit end face.

Even in an ideal case in which a perfect diffusion surface 220exhibiting a predetermined luminance value independently of the angle isarranged on an incident end face 211 of a random light guide 210 havingthe above-described optical nature (FIG. 9), the angular characteristicsof luminance on the exit side are not constant and degraded toward themaximum incident angle, as indicated by a solid line in FIG. 10.

In this case, a maximum incident angle θ₀ is obtained from a numericalaperture determined by the following equation: ##EQU1## where n1 is therefractive index of the core of the optical fiber, and n2 is therefractive index of the clad.

Therefore, to obtain angular characteristics exhibiting a predeterminedluminance up to an angle (necessary angle) according to a desiredillumination field, the angular characteristics of luminance on theincident side must represent a complementary shape with respect to thatindicated by the solid line in FIG. 10, i.e., angular characteristicsindicated by a dotted line in FIG. 10. In the present invention, adiffusion means is used to spread the angle of the parallel beam(angular characteristics of luminance are shown in FIG. 11). At the sametime, the parallel beam is caused to be incident on the incident end ofthe light guide with an angle. With this arrangement, angularcharacteristics shown in FIG. 12, which are close to the angularcharacteristics of luminance indicated by the dotted line in FIG. 10,can be obtained. Therefore, angular characteristics exhibiting an almostpredetermined luminance up to the necessary angle can be obtained on theexit side.

More specifically, when a laser beam is to be incident on the incidentend face 211 of the random light guide 210 through a diffusion plate 213serving as a diffusion means, the incident end face 211 of the randomlight guide 210 is arranged to be inclined by an angle θ with respect toa plane perpendicular to a direction of incidence of the laser beam ontothe diffusion plate 213. In this case, the distribution of light amountson an exit end face 212 can be considered to be uniform because of thenature of the random light guide. For this reason, a secondary sourcehaving perfect diffusion properties equal to those of a perfectdiffusion surface up to the necessary angle can be formed at the exitend of the random light guide 210. At this time, since the light amountof a portion beyond the necessary angle can be minimized, the energyloss in the entire optical system can be further decreased.

In the illumination optical system of the present invention, when aninclined parallel beam is incident from the incident side of the randomlight guide, as described above, the optical nature of the random lightguide is utilized, i.e., the angle on the exit side is almost maintainedthrough it is slightly diffused, and the directions are distributed atrandom. When the diameter of the incident laser beam is caused tocoincide with that of the incident end face 211 of the random lightguide, efficient energy transmission can be performed.

By using the secondary source formed in this manner, a desiredillumination field having a uniform illuminance distribution and apredetermined illumination numerical aperture in the illumination fieldis formed by a condenser lens 214.

As described above, according to the present invention, an illuminationfield having a uniform illuminance distribution and a predeterminedillumination numerical aperture in the illumination field can beobtained with a simple arrangement while ensuring a high degree offreedom in light guide.

The inclination angle θ of the incident end face of the light guide withrespect to a plane perpendicular to a direction of incidence of theparallel beam onto the diffusion means preferably satisfies 4°≦θ≦10°.When the inclination angle θ is smaller than 4°, the angularcharacteristics of luminance indicated by the dotted line in FIG. 10,which are complementary with the angular characteristics of luminanceindicated by the solid line, cannot be obtained on the incident sidebecause of a short of inclination. The angular characteristics ofluminance at the exit end of the light guide are rapidly degraded towardthe maximum incident angle, as shown in FIG. 15, so predeterminedangular characteristics of luminance up to the necessary angle cannot beobtained.

When the inclination angle is larger than 10°, the inclination becomestoo large, and the luminance is degraded within the range up to thenecessary angle. FIG. 16 is a graph showing a case in which theinclination angle θ is set to 12°. The angular characteristics ofluminance represent a peak value near the maximum incident angle θ₀. Theluminance becomes very low within the range up to the necessary angle.Therefore, when the above condition is satisfied, angularcharacteristics exhibiting a predetermined high luminance up to thenecessary angle can be obtained at the exit end of the light guide.

The present invention will be described below in accordance with theembodiments.

(First Embodiment)

An illumination optical system used for an ISS (Image Slit Sensor)system alignment apparatus of a projection exposure apparatus is shownin FIG. 17 as the first embodiment of the present invention. In thisexposure apparatus, an exposure light beam output from an excimer lasersource 100 is adjusted by a beam expander 101 into a parallel beamhaving an appropriate diameter and incident on a reticle 11 through amain illumination system constituted by a fly-eye integrator 113, arelay lens 114, a mirror 115, and a condenser lens 116. The exposurelight beam transmitted through the reticle 110 is irradiated on a wafer(not shown) mounted on a wafer table on a stage through a projectionlens 109, thereby projecting the pattern of the reticle 110 onto thewafer surface.

The alignment apparatus of this embodiment, which detects the positioncoordinates of the reticle 110, commonly uses the excimer laser source100 for exposure. In an alignment operation, a switching mirror 102 isinserted between the laser source 100 side and the main illuminationsystem, thereby guiding the laser beam from the light source to theillumination optical system of the alignment apparatus. The illuminationoptical system is constituted by a diffusion plate 104, a random lightguide 105, and a condenser lens 106.

In the above arrangement, the laser beam from the excimer laser source100 is reflected by the switching mirror 102 through the beam expander101 and incident on the alignment illumination optical system. In theillumination optical system, the parallel beam adjusted to have anappropriate diameter by a beam expander 103 is diffused by the diffusionplate 104 and incident on the random light guide 105. At this time, theincident end face of the random light guide 105 is arranged to have apredetermined inclination angle θ with respect to a plane perpendicularto the direction of incidence of the parallel beam onto the diffusionplate 104. In this case, the parallel beam is perpendicularly incidenton the surface of the diffusion plate 104. The incident end face of therandom light guide 105 is arranged to be inclined by θ=about 7° withrespect to the surface of the diffusion plate 104.

With this arrangement, the angular characteristics of luminance shown inFIG. 12 can be obtained at the incident end of the random light guide105. The light beam emerges from the exit end with predetermined angularcharacteristics exhibiting a predetermined luminance up to the necessaryangle, as shown in FIG. 13. The illumination light emerging from therandom light guide 105 forms a secondary source and illuminates a markSM on an index plate 108 arranged at the same position as the wafersurface on the stage (on the image formation surface of the projectionlens 109) from the inside of the stage with a uniform illuminancedistribution in an appropriate illumination field through a mirror 107.

In this embodiment, the mark SM is a transparent slit. When this mark SMis illuminated from the lower side, illumination light emerges from theslit. A reticle mark RM is arranged on the reticle 110 which is locatedat a position conjugate with the index plate 108 with respect to theprojection lens 109 telecentric on both sides. This mark RM is also atransparent slit.

When the reticle 110 optically overlaps the index plate 108 on the stagethrough the projection lens 109, the light beam from the mark SM istransmitted through the projection lens 109 and then the reticle mark RMand reversely guided through the main illumination system for exposure.The light beam is guided to a detection optical system along the opticalpath deflected by a switching mirror 111 (inserted into the optical pathonly during alignment) and is detected by a detector (photomultipliertube) 112.

When the reticle mark RM and the mark SM are relatively moved inaccordance with the movement of the stage, the light amount to bedetected by the detector 112 changes (FIG. 18). The driving system iscontrolled to move the stage such that the detection light amount ismaximized, i.e., the reticle mark RM and the mark SM overlap each otherthrough the projection lens 109. The stage coordinates at a positionwhere the detection light amount is maximized can be obtained by aninterferometer or the like.

As shown in FIG. 19, when a plurality (three in this case) of reticlemarks RM are arranged at positions separated from the reticle center onthe reticle 110 by a predetermined distance, and the stage coordinatesare measured for each reticle mark RM, the stage conversion coordinatesof the reticle center can be obtained. To obtain the same degree ofspreading of the light beam in the projection lens 109 as in an exposureoperation, the illumination numerical aperture by the alignmentillumination optical system is preferably equalized with that convertedon the wafer of the main illumination system for exposure from theviewpoint of precision.

(Second Embodiment)

An illumination optical system used for a reticle alignment systemalignment apparatus of a projection exposure apparatus is shown in FIG.20 as the second embodiment of the present invention. This exposureoptical system is the same as that of the first embodiment. An exposurelight beam output from an excimer laser source 120 is adjusted by a beamexpander 121 into a parallel beam having an appropriate diameter andincident on a reticle 130 through a main illumination system constitutedby a fly-eye integrator 134, a relay lens 135, a mirror 136, and acondenser lens 134. The exposure light beam transmitted through thereticle 130 is irradiated on a wafer (not shown) mounted on a wafertable on a stage through a projection lens 129, thereby projecting thepattern of the reticle 130 onto the wafer surface.

The alignment apparatus of this embodiment, which performs alignment ofthe reticle 130, commonly uses the excimer laser source 120 forexposure. In an alignment operation, a switching mirror 122 is insertedbetween the laser source 120 side and the main illumination system,thereby guiding the laser beam from the light source to the illuminationoptical system of the alignment apparatus. The illumination opticalsystem is constituted by a diffusion plate 124, a random light guide125, and a condenser lens 126.

In the above arrangement, the laser beam from the excimer laser source120 is reflected by the switching mirror 122 through the beam expander121 and incident on the alignment illumination optical system. In theillumination optical system, the parallel beam adjusted to have anappropriate diameter by a beam expander 123 is diffused by the diffusionplate 124 and incident on the random light guide 125. At this time, theincident end face of the random light guide 125 is arranged to have apredetermined inclination angle θ with respect to a plane perpendicularto the direction of incidence of the parallel beam onto the diffusionplate 124. In this case, the parallel beam is perpendicularly incidenton the surface of the diffusion plate 124. The incident end face of therandom light guide 125 is arranged to be inclined by θ=about 7° withrespect to the surface of the diffusion plate 124.

With this arrangement, the angular characteristics of luminance shown inFIG. 12 can be obtained at the incident end of the random light guide125. The light beam emerges from the exit end with predetermined angularcharacteristics exhibiting a predetermined luminance up to the necessaryangle, as shown in FIG. 13. The illumination light emerging from therandom light guide 125 forms a secondary source and illuminates a markSM on an index plate 128 arranged at the same position as the wafersurface on the stage (on the image formation surface of the projectionlens 129) from the inside of the stage with a uniform illuminancedistribution in an appropriate illumination field through a mirror 127.

A transparent mark SM is formed on the index plate 128. When this markSM is illuminated from the lower side, illumination light emergestherefrom. A transparent cross reticle mark RM as shown in FIG. 21 isarranged on the reticle 130 which is located at a position conjugatewith the index plate 128 with respect to the projection lens 129telecentric on both sides.

When the reticle 130 is at a predetermined position with respect to theindex plate 128 on the stage, light from the mark SM is transmittedthrough the reticle mark RM through the projection lens 129, and theoptical path is deflected by a mirror 131 arranged to be inclined by45°. The light beam is incident on a CCD camera 133 through an objectivelens 132, and image detection of the reticle mark RM is performed. Onthe basis of the image detection result, alignment can be performed suchthat the center of the reticle mark RM is located at the center of theoptical axis of the objective lens 132.

As shown in FIG. 22, for each reticle used in this exposure apparatus, areticle mark with respect to the reticle center is standardized. In thiscase, even when the reticle is exchanged, the reticle position can bereproduced such that the reticle center is always located at the sameposition. To form a satisfactory image intensity distribution when theimage of the reticle mark RM is focused on the light-receiving surfaceof the CCD camera 133, an appropriate illumination numerical aperture bythe illumination system is required.

In the illumination optical system of the first or second embodiment,the parallel beam is perpendicularly incident on the diffusion platesurface, and the incident end face of the random light guide is arrangedto be inclined by the predetermined angle θ with respect to thediffusion plate surface. FIG. 23 is a view showing a modification of theillumination optical system using another method in which the incidentend face of the random light guide is arranged to be inclined by thepredetermined inclination angle θ with respect to a plane perpendicularto the direction of incidence of the parallel beam.

An incident end face 144 of a random light guide 143 is arranged to beparallel to the surface of a diffusion plate 142. A parallel beamsupplied from a light source is incident to be inclined by apredetermined angle with respect to a direction perpendicular to thesurface of the diffusion plate 142 through a Keplerian relay lens system141. In this case, the lens of the Keplerian relay lens system 141 onthe light source side is eccentrically arranged with respect to the lenson the diffusion plate 142 side, thereby inclining the exit direction ofthe light beam from the Keplerian relay lens system 141 with respect tothe incident direction to the relay lens system.

Therefore, the illumination light has angular characteristics ofluminance as shown in FIG. 13 at an exit end 145 of the random lightguide 143 to form an appropriate illumination field having a uniformilluminance distribution through a condenser lens 146.

FIG. 24 is a view showing another modification of the illuminationoptical system. As in FIG. 23, an incident end face 154 of a randomlight guide 153 is arranged to be parallel to the surface of a diffusionplate 152. In place of the eccentric lens on the light source side inFIG. 23, a fly-eye lens 150 (four-lens structure) is used to incline theexit direction of the light beam from a lens 151 with respect to theincident direction to the fly-eye lens 150. More specifically, theincident direction to the diffusion plate 152 is inclined with respectto a direction perpendicular to the diffusion plate 152. Because of itseffect for eliminating variations in light amount on the incident endface 154 of the random light guide 153, this illumination optical systemhas a higher resistance to variations in illuminance of an incidentlaser beam.

In the illumination field formed by the above illumination opticalsystem, the diffusion effect of the diffusion plate and the angulardiffusion effect of the random light guide are provided. In addition,the positional relationship on the incident side by the randomarrangement of the optical fibers in the random light guide is notmaintained. Because of the synergistic effect of these effects, nospeckle or interference fringe is generated particularly when an excimerlaser which is not so good in coherency is used.

FIG. 25 is a view showing, as a reference, an illumination opticalsystem for mainly obtaining a desired illumination field by only theangular diffusion effect of a random light guide 163. In this case, afly-eye lens 161 is used as a diffusion means on the incident side.However, since the angular characteristics are not spread by thediffusion plate unlike the above embodiments, angular characteristicsexhibiting a predetermined luminance up to a necessary angle, as shownin FIG. 13, cannot be obtained. For this reason, this system is slightlydisadvantageous in illuminance variations of the illumination field.Such an illumination optical system can be used when angularcharacteristics of predetermined luminance are not so strictly required.In this case, a relay lens system may be used in place of the fly-eyelens 161.

In each of the above embodiments, the diffusion plate is used as adiffusion means. However, the present invention is not limited to this.A phase grating or a diffuser can also be used as far as it has adiffusion effect. Any diffusion plate can be used as far as it canobtain the illuminance distribution shown in FIG. 11, and ground glassor a lemon skin filter which is chemically treated can also be used. Inaddition, in each of the above embodiments, the illumination opticalsystem for the alignment apparatus of a projection exposure apparatushas been described. The illumination optical system according to thepresent invention can also be used for another optical system, andparticularly for an optical system using a light source with a smallillumination numerical aperture, such as an excimer lasers and requiringthe degree of freedom for supplying light.

An embodiment of an alignment apparatus according to the presentinvention will be described below.

FIG. 26 is a view showing the main part of a projection exposureapparatus having a stage light emission type reticle alignment systemand a reticle position detection system. Referring to FIG. 26, a reticle1 is held on a reticle stage 2. The pattern of the reticle 1 isprojected and exposed in each shot area on a wafer 4 through aprojection optical system 3 with continuously emitted exposure light(not shown) (e.g., the i-ray or g-ray). In this case, the Z-axis is setto be parallel to an optical axis AX of the projection optical system 3,and the X- and Y-axes are respectively set in directions parallel andperpendicular to the sheet surface of FIG. 26 within a planeperpendicular to the optical axis AX. Cross reticle marks 10A and 10Bare formed at the two ends of the pattern area of the reticle 1 alongthe X direction. Reticle alignment systems 15A and 15B are arrangedabove the reticle marks 10A and 10B, respectively.

The wafer 4 is mounted on a wafer stage 6 through a wafer holder 5. Thewafer stage 6 is constituted by an X-Y stage for positioning the wafer 4in an X-Y plane, a Z stage for positioning the wafer 4 in the Zdirection, a leveling stage for correcting the inclination angle of thewafer 4, and the like. The X- and Y-coordinates of the wafer stage 6 arealways measured by laser interferometers (not shown), respectively. Atransparent stage base 7 is attached near the wafer holder 5 on thewafer stage 6. In the light-shielding film on the upper surface of thestage base 7, a pair of reference marks 8A and 9A and a pair ofreference marks 8B and 9B each constituted by a rectangular opening areformed in two areas which are, e.g., symmetrical with each other aboutthe center along the X-axis, as shown in FIG. 27.

Referring back to FIG. 26, in reticle alignment, the reference marks 8Aand 9A, and 8B and 9B are illuminated from the bottom surface side bythe stage light emission type illumination system. The illuminationlight beams passing through the reference marks 8A and 8B respectivelyilluminate reticle marks 10A and 10B on the lower surface of the reticle1 through the projection exposure system 3. Depending on thespecifications of the projection area of the projection optical systemto be used, inner reticle marks (to be referred to as reticle marks 11Aand 11B) are used in some cases. For this reason, the illumination lightbeams passing through the reference marks 9A and 9B respectivelyilluminate the reticle marks 11A and 11B through the projection opticalsystem 3. More specifically, when the center of the stage base 7 matchesthe optical axis AX, the reference marks 8A and 8B are almost conjugatewith the reticle marks 10A and 10B, and the reference marks 9A and 9Bare almost conjugate with the reticle marks 11A and 11B.

In the stage light emission type illumination system, continuouslyemitted illumination light in the same wavelength band as that of theexposure light guided by the light guide from an exposure light source(not shown) emerges from exit ends 12a and 12b of the light guide intothe wafer stage 6. The illumination light emerging from the exit end 12ais vertically reflected upward by a mirror 13A to illuminate thereference marks 8A and 9A through a condenser lens 14A. Similarly, theillumination light emerging from the exit end 12b illuminates thereference marks 8B and 9B through a mirror 13B and a condenser lens 14B.At this time, when the center of the stage base 7 matches the opticalaxis AX, the images of the reference marks 8A, 8B, and the like areformed to surround the reticle marks 10A, 10B, and the like because theillumination light is in the same wavelength band as that of theexposure light. The reticle marks are illuminated by these images. Thestage light emission type illumination system has the above-describedarrangement.

FIG. 28 is a plan view showing the reticle alignment system in FIG. 26.In a reticle alignment system 15A on the right side of FIG. 28, theillumination light transmitted through the periphery of the reticle mark10A (or 11A) on the reticle 1 is reflected by a mirror 16A fordeflecting the optical path and incident on a half mirror 20A through afirst objective lens 17A, a mirror 18A, and a second objective lens 19A.The light beam reflected by the half mirror 20A forms a synthetic imageof the reference mark 8A and the reticle mark 10A on the image pickupsurface of a two-dimensional image pickup element 21A consisting of atwo-dimensional CCD or the like. Misalignment of the reticle mark 10A isdetected by, e.g., visual observation using the reference mark 8A as anindex mark from two-dimensional image data from the two-dimensionalimage pickup element 21A. In addition, when image processing of thetwo-dimensional image data is performed, the positional shift isdetected as those of the X- and Y-coordinates on the wafer stage.

The light beam transmitted through the half mirror 20A forms the imageof the reticle mark 10A subjected to transmission illumination with theillumination light using the image of the reference mark 8A as anillumination field on a vibration slit 23A in a two-dimensionalphotoelectric microscope 22A.

A Y-axis photoelectric microscope in the two-dimensional photoelectricmicroscope 22A is constituted by the vibration slit 23A vibrating in adirection corresponding to the Y direction on the image surface of thereticle mark 10A, and a photoelectric detector 24A such as aphotomultiplier arranged immediately subsequent to the vibration slit.When an output signal from the photoelectric detector 24A issynchronously rectified by a driving signal of the vibration slit 23A, asignal corresponding to the Y-direction shift of the reticle mark 10Awith respect to a predetermined reference position fixed with respect tothe reticle stage 2 can be obtained. Although not illustrated, an X-axisphotoelectric microscope constituted by a vibration slit vibrating in adirection corresponding to the X direction on the image surface of thereticle mark 10A, and a photoelectric detector arranged immediatelysubsequent to the vibration slit is also included in the photoelectricmicroscope 22A. With this X-axis photoelectric microscope, theX-direction positional shift of the reticle mark 10A from apredetermined reference position can be detected,

A reticle alignment system 15B on the left side is also constituted by amirror 16B to a two-dimensional photoelectric microscope 22B to besymmetrical with the right-side alignment system. The positional shiftof the reticle mark 10B can be detected by a two-dimensional imagepickup element 21B of the reticle alignment system 15B by using theother reference mark 8B as an index mark. At the same time, the X- andY-direction shifts of the reticle mark 10B from a predeterminedreference position can be detected by the photoelectric microscope 22B.

As a result, when image pickup signals from the two two-dimensionalimage pickup elements 21A and 21B are processed, the positionalrelationship of the reticle 1 with respect to the wafer stage 6(two-dimensional shift and a rotation angle) can be obtained. Inaddition, the two-dimensional shifts of the two reticle marks 10A and10B can be detected by the two photoelectric microscopes 22A and 22B.For this reason, the X- and Y-direction shifts and the rotation angle ofthe reticle 1 with respect to the reticle stage 2 can be obtained. Wheneach of the X- and Y-direction shifts and the rotation angle fallswithin an allowance, reticle alignment is completed.

When the inner reticle marks 11A and 11B of the reticle 1 are to be usedin accordance with the projection area of the projection optical system3 to be used, the first objective lens 17A and the mirror 16A are movedtogether in the -X direction. At the same time, a first objective lens17B and the mirror 16B are moved together in the +X direction. With thisoperation, the mirrors 16A and 16B must be located above the reticlemarks 11A and 11B.

At this time, as disclosed in Japanese Patent Laid-Open No. 57-142612,the lower surface (pattern surface) of the reticle 1 is located on theimage formation planes of the first objective lenses 17A and 17B throughthe mirrors 16A and 16B. Even when the mirrors 16A and 16B and the firstobjective lenses 17A and 17B are moved in the X direction along thereticle 7, the lower surface of the reticle 1 always match the imageformation planes of the first objective lenses 17A and 17B. According tothis arrangement, a parallel system is always formed between the firstobjective lens 17A (or 17B) and the second objective lens 19A (or 19B).Regardless of the above movement, the conjugate relationship between thepattern surface of the reticle 1 and the two-dimensional image pickupelement, and the vibration slit of the two-dimensional photoelectricmicroscope, and the magnification are maintained. Therefore, the reticlealignment system 15A and 15B can cope with use of the reticle marks 11Aand 11B, and the reference marks 9A and 9B.

As described above, in the reticle alignment system shown in FIG. 26,the stage light emission type illumination system using continuouslyemitted illumination light in the same wavelength band as that of theexposure light is used to illuminate the reference marks and the reticlemarks. For this reason, position detection is accurately performed byboth the two-dimensional image pickup elements 21A and 21B and thephotoelectric microscopes 22A and 22B.

To cope with a further improvement of the degree of integration of asemiconductor element or the like, a projection exposure apparatus whichuses exposure light having a shorter wavelength to improve theresolution has also been used in recent years. As current exposure lighthaving a short wavelength as in the far-ultraviolet range, an excimerlaser beam such as a KrF excimer laser beam (wavelength: 248 nm) and anArF excimer laser beam (wavelength: 193 nm), a metal vapor laser beam,or a light beam such as a pulsed harmonic of a YAG laser is mainly used.

However, assume that the reference marks and the reticle marks areilluminated by the stage light emission method using pulsed illuminationlight branched from pulsed exposure light. In this case, no high-qualitysignal can be obtained when the detection signal is synchronouslyrectified by the driving signal of the vibration slit in thephotoelectric microscope 22A or 22B. Therefore, reticle alignment by thephotoelectric microscopes 22A and 22B is not accurately performed.

(Third Embodiment)

An embodiment of an alignment apparatus according to the presentinvention will be described below with reference to FIGS. 29 to 33. Inthis embodiment, the present invention is applied to the reticlealignment system of a projection exposure apparatus using, as exposurelight, a pulsed far-ultraviolet beam. The same reference numerals as inFIGS. 26 to 28 denote the same elements in FIGS. 29 to 30, and adetailed description thereof will be omitted.

FIG. 30 is a view showing the projection exposure apparatus of thepresent invention. Referring to FIG. 30, exposure light in thefar-ultraviolet range (e.g., wavelength: 248.4 nm) is pulsed and emittedfrom a pulse light source 31 consisting of, e.g., a KrF excimer lasersource. The sectional shape of the pulsed exposure light beam (pulseexposure light) is spread by a beam expander 32, and the light beam isincident on a fly-eye lens 34. In this embodiment, a mirror 33 forswitching the optical path is arranged to be inclined by 45° withrespect to the optical axis and freely inserted between the beamexpander 32 and the fly-eye lens 34. During exposure, the mirror 33 isretreated to a position 33A. During reticle alignment, the mirror 33 isset in the optical path of the exposure light.

In the exposure operation, a pulse exposure light beam IL from asecondary source formed on the rear-side (reticle-side) image formationplane of the fly-eye lens 34 is reflected by a mirror 35 and illuminatesthe pattern area of the lower surface (pattern surface) of a reticle 1with a uniform illuminance distribution through a first relay lens 36, avariable field stop (reticle blind) 37, a second relay lens 38, and amain condenser lens 39. With the pulse exposure light beam IL, thepattern image of the reticle 1 is projected and exposed on each shotarea of a wafer 4 on a wafer stage 6 through a projection optical system3.

On the other hand, in reticle alignment, the mirror 33 is set in theoptical path of the pulse exposure light beam. The light beam (to bereferred to as a pulse illumination light beam hereinafter) reflected bythe mirror 33 is supplied to an incident end 12c of a light guide 12through a beam expander 40 for reducing the beam diameter. Two exit ends12a and 12b of the light guide 12 are inserted in the wafer stage 6.Pulse illumination light beams BL emerging from the exit ends 12a and12b illuminate reference marks 8A and 9A, and 8B and 9B each constitutedby a rectangular opening and formed in the upper surface of a stage base7 through mirrors 13A and 13B and condenser lenses 14A and 14B,respectively. When the center of the stage base 7 almost matches anoptical axis AX of the projection optical system 3, the pulseillumination light beams BL passing through the reference marks 8A and9A, and 8B and 9B form the images of the reference marks near reticlemarks 10A and 11A, and 10B and 11B on the pattern surface of the reticle1 through the projection optical system 3, respectively. In FIG. 30,reticle alignment systems 41A and 41B are arranged above the reticlemarks 10A and 10B, respectively.

FIG. 29 is a plan view showing the reticle alignment systems of thisembodiment. In the reticle alignment system 41A on the right side ofFIG. 29, a continuous light beam (to be referred to as a "continuousillumination light beam" hereinafter) AL in a wavelength range (e.g.,red or near-infrared light) different from that of the pulse exposurelight beam IL is selected from light continuously emitted from a halogenlamp 42A by an optical filter (not shown). A light-emitting diode or thelike can also be used in place of the halogen lamp 42A. The continuousillumination light beam AL is irradiated on the opening of a field stop46A through a condenser lens 43A, a light guide 44A, and a condenserlens 45A, as indicated by a dotted line.

The continuous illumination light beam AL passing through the opening ofthe field stop 46A is reflected by a half mirror 47A and incident on adichroic mirror 50A through a second objective lens 48A and a correctionlens 49A. The dichroic mirror 50A has wavelength selection propertiesfor reflecting the pulse illumination light beam BL and transmitting thecontinuous illumination light beam AL. Therefore, the continuousillumination light beam AL is transmitted through the dichroic mirror50A and illuminates the reticle mark 10A through a first objective lens51A and a mirror 52A.

Referring to FIG. 30, movable mirrors 56A and 56B are arranged on thebottom surface side of a reticle stage 2. The movable mirrors 56A and56B are supported to be movable along the lower surface of thereticle 1. If the reflectance of the reticle mark 10A or 10B is low, themovable mirrors 56A and 56B are arranged below the reticle marks 10A and10B, respectively. At this time, the movable mirror 56A is arranged on aplane conjugate with the arrangement plane of the field stop 46A in FIG.29. The reason for this will be described later in detail.

The continuous illumination light beam AL reflected by the reticle mark10A, the continuous illumination light beam AL reflected by the movablemirror 56A in the set state and passing through the periphery of thereticle mark 10A, and the pulse illumination light beam BL passingthrough the periphery of the reticle mark 10A are guided to the dichroicmirror 50A through the mirror 52A and the first objective lens 51A.Referring back to FIG. 30, the pulse illumination light beam BL isreflected by the dichroic mirror 50A and forms the images of thereference mark 8A and the reticle mark 10A on the image pickup surfaceof a two-dimensional image pickup element 21A constituted by atwo-dimensional CCD or the like through a mirror 53A and a secondobjective lens 54A. On the other hand, the continuous illumination lightbeam AL is transmitted through the dichroic mirror 50A and forms theimage of the reticle mark 10A on a vibration slit 23A of atwo-dimensional photoelectric microscope 22A trough the correction lens49A, the second objective lens 48A, and the half mirror 47A. Althoughnot illustrated, a vibration slit having a vibration directionperpendicular to that of the vibration slit 23A and an immediatelysubsequent photoelectric detector are also included in the photoelectricmicroscope 22A.

When an image pickup signal from the two-dimensional image pickupelement 21A is processed, the X- and Y-direction shifts of the reticlemark 10A with respect to the reference mark 8A are measured.Additionally, the X- and Y-direction shifts of the reticle mark 10A withrespect to the reference point (reticle stage 2) in the photoelectricmicroscope 22A are measured by the two-dimensional photoelectricmicroscope 22A. A movable optical system 55A constituted by thecorrection lens 49A to the mirror 53A is arranged to be movable in the Xdirection. When position of, e.g., the reticle mark 10A is to bedetected, the movable optical system 55A moves such that the mirror 52Ais located above the reticle mark 11A.

If the pulse illumination light beam BL is a light beam branched fromthe exposure light from the KrF excimer laser source for pulsing lightin the far-ultraviolet range (e.g., wavelength: 248.8 nm), the glassmaterial usable for the pulse illumination light beam BL is limited. Forthis reason, when the lower surface of the reticle 1 matches the imageformation plane of the first objective lens 51A for the pulseillumination light beam BL, the focal point of the first objective lens51A is located below the reticle 1 for the continuous illumination lightbeam AL having a longer wavelength. In this state, a parallel system isformed between the first objective lens 51A and the second objectivelens 54A for the pulse illumination light beam BL while a divergencesystem is formed between the first objective lens 51A and the secondobjective lens 48A for the continuous illumination light beam AL asnonexposure light.

To prevent this, the correction lens 49A for correcting the focal pointof the first objective lens 51A for the continuous illumination lightbeam AL to be on the lower surface of the reticle 1 is arranged on theoptical path of the continuous illumination light beam AL transmittedthrough the dichroic mirror 50A, thereby moving the first objective lens51A and the correction lens 49A together. Therefore, a parallel systemis formed between the correction lens 49A and the second objective lens48A. Even when the movable optical system 55A moves in the X directionas indicated by the dotted line in FIG. 29, the conjugate relationshipbetween the lower surface of the reticle 1 and the vibration slit of thephotoelectric microscope 22A and the focusing magnification aremaintained in the initial state. Therefore, even the reticle mark 11A ata different position can also be accurately measured.

The reticle alignment system 41B on the left side is also constituted bya halogen lamp 42B to a second objective lens 54A, a two-dimensionalimage pickup element 21B, and a photoelectric microscope 22B to besymmetrical with the right-side alignment system. The positionalrelationships of the reticle marks 10B and 11b with respect to thereticle stage 2, and the positional relationships of the reticle marks10B and 11B with respect to the reference marks 8B and 9B are measuredby the reticle alignment system 41B.

The function of the movable mirror 56A in FIG. 30 will be describedbelow. When incident illumination is used for the continuousillumination light beam AL, as in this embodiment, the reflectance ofthe reticle mark 10A or 11A always poses a problem. In this embodiment,however, the problem caused by the reflectance is solved by thefollowing method. A pattern obtained when a chromium film formed on aglass substrate is etched to leave a cross mark-like chromium film isnormally used as the reticle mark 10A or 11A. There are some patternswhose reflectance for a light beam incident from the upper surface islower than that of the normal pattern (reflectance: about 40%).

When the circuit pattern in the pattern area of the reticle 1 has such alow reflectance, the reticle mark inevitably has a low reflectancebecause only the reflectance of the reticle mark cannot be changed. Inthis case, however, since the alignment system shown in FIG. 26 usesonly stage light emission, the reticle mark 10A or 11A is subjected totransmission illumination, so the detection precision does not depend onthe reflectance. However, the reticle alignment system 41A of thisembodiment performs incident-light illumination from the upper side. Forthis reason, if the reflectance of the reticle mark is low, the lightintensity of the obtained image is decreased. In the worst case, theimage of the reticle mark cannot be obtained. In such a case, accordingto this embodiment, the movable mirror 56A is moved to the lower side ofthe reticle mark having a low reflectance to reflect the incidentillumination light beam transmitted through the reticle mark once,thereby performing uniform transmission illumination of the reticle markfrom the lower side.

The illumination state when the movable mirror 56A parallel to thereticle 1 is inserted below the reticle 1 as needed will be describedbelow with reference to FIGS. 31 and 32.

FIG. 31 is a view showing the optical path of the continuousillumination light beam AL having an illumination field with a uniformilluminance distribution on a plane 57C. A rectangular distribution 58Crepresents the illuminance distribution on the plane 57 with an opticalaxis 61 at its center. The illumination area gradually spreads towardplanes 57B and 57A which are separated upward from the plane 47C alongthe optical axis 61 (to be defined as a negative direction). Similarly,the illumination field gradually spreads toward planes 57D and 57E whichare separated downward from the plane 57c along the optical axis 61 (inthe positive direction). The illuminance distributions on the planes 57Ato 57E are shown as distributions 58A to 58E on the right side,respectively. It is found that the uniform illuminance area (area with aflat illuminance distribution) is decreased along with an increase indistance from the plane 57C as a reference in the positive or negativedirection. A hatched area 62 indicates the uniform illuminance area.

The diameter of the uniform illuminance area is defined as Φ_(E). Asshown in FIG. 31, assume an ideal case in which the light beam has asymmetrical shape above and below the plane 57C. If an openinghalf-angle θ of the continuous illumination light AL is small, thenumerical aperture NA can be approximated as NA=sinθ≈θ. In this case,the following relation is satisfied:

    Φ.sub.E =Φ-2dθ.

where θ is the diameter of the illumination field, and d is the distancefrom the illumination field in the upward or downward directions (Zdirection).

This nature is compatible with a reticle mark having a low reflectance.

FIG. 32 is a view showing a state in which the movable mirror 56A isarranged below the reticle mark 10A. Referring to FIG. 32, the movablemirror 56A is arranged at a position separated from the lower surface ofthe reticle 1 by a distance L to be parallel to the reticle 1. Theillumination field of incident-light illumination with the continuousillumination light beam AL corresponds to the mirror surface of themovable mirror 56A. The light beam reflected by the movable mirror 56Aindicated by a chain double-dashed line and the reticle 1M as a mirrorimage are illustrated below the movable mirror 56A. For the normalreticle 1, the movable mirror 56A is not arranged, and the illuminancedistribution of the reticle mark 10A is represented by a distribution60A having a moderate periphery.

In this case, if the uniform illuminance field in the distribution 60Ais larger than the actually observed field, uniform illumination isperformed. When a reticle having a low reflectance is used, the movablemirror 56A is inserted, as in FIG. 32, thereby illuminating the reticle1M as a mirror image. That is, a reticle mark 10AM as the mirror imageof the reticle mark 10A is subjected to transmission illumination. Atthis time, it must be taken into consideration how the illuminancechanges upon eclipse of the light beam indicated by a solid line by theactual reticle mark 10A.

FIG. 33 is an enlarged perspective view of the reticle mark 10AM as amirror image and the reticle mark 10A in FIG. 32. Referring to FIG. 33,the line width of the reticle mark 10A is defined as Δ, and theilluminance on the lower surface of the reticle 1 when no eclipse ispresent, i.e., when Δ=0, is defined as E₀. The interval between themovable mirror 56A and the reticle 1 is defined as L and the numericalaperture NA by the continuous illumination light beam AL is defined assinθ≈θ, an actual illuminance E at this time satisfies the followingrelation:

    E≈E.sub.0 {1-2Δ/(πLθ)}

If each variable can be set such that E≈E₀ can be considered, theilluminance variations caused by the eclipse by the reticle mark 10Aneed not be taken into consideration. Even in transmission illuminationwith the light beam reflected by the movable mirror 56A, an illuminancedistribution as the distribution 60A can be obtained. At this time,since the image of the reticle mark 10A by incident illumination ishardly obtained, reticle alignment can be performed using the image bytransmission illumination with the light beam reflected by the movablemirror 56A. Note that the state in which E≈E₀ can be consideredcorresponds to a state in which (E-E₀) is several percent or less of E₀,i.e., 2Δ/(πLθ) becomes several percent or less.

As described above, in this embodiment, conventional reticle alignmentcan be executed for all reticles only by adding a simple mirrormechanism. Additionally, when a normal reticle is to be used, themovable mirrors 56A and 56B need not be moved, resulting in a higherthroughput.

Although not illustrated, an illumination system of the continuousillumination light AL as a nonexposure light may be arranged below thereticle 1 in place of the movable mirrors 56A and 56B, and illuminationmay be performed from the lower side as needed. This arrangement cancope with a reticle having any reflectance. However, a spacial margin isrequired between the reticle 1 and the projection optical system 3, andthe illumination system must be inserted or retreated when a normalreticle is used.

In addition, the illumination field is arranged on the surface of themovable mirror when the movable mirror is located below the reticle.However, this position may be slightly shifted as far as the problem ofperformance is not posed. If the problem of performance is not posed,the actual illumination field at the position of the reticle mark mayslightly protrude from the uniform illuminance area. A dichroic mirrorfor reflecting the continuous illumination light beam AL andtransmitting the pulse illumination light beam BL (exposure light) maybe used in place of the movable mirrors 56A and 56B.

FIG. 34 is a view showing the projection optical apparatus shown in FIG.30, in which the light guide 12 is replaced with the random light guide105 shown in FIG. 17, and the diffusion plate 104 is arranged at theincident end. Similarly, a light beam may be supplied to the incidentend of the light guide 12 shown in FIG. 30 by the arrangement shown inFIGS. 23 to 25.

From the invention thus described, it will be obvious that the inventionmay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedfor inclusion within the scope of the following claims.

The basic Japanese Application Nos. 6-132395 (132395/1994) filed on May24, 1994 and 6-212684 (212684/1994) filed on Sep. 6, 1994 are herebyincorporated by reference.

What is claimed is:
 1. An alignment apparatus for aligning a mask withan alignment mark and a photosensitive substrate on a substrate stage,comprising:a first alignment illumination system for illuminating saidalignment mark and a reference mark on said substrate stage with a pulseillumination light obtained upon branching a pulsed exposure light; asecond alignment illumination system for illuminating said alignmentmark with a continuously emitted continuous illumination light in awavelength range different from that of the exposure light; a firstobjective optical system for focusing the continuous illumination lightfrom said alignment mark, the pulse illumination light from saidalignment mark, and the pulse illumination light from said referencemark through a projection optical system; a wavelength selection opticalsystem for dividing a light beam focused by said first objective opticalsystem into a first light beam of the continuous illumination light anda second light beam of the pulse illumination light; a second objectiveoptical system for forming an image of said alignment mark from saidfirst light beam; an image position detector, having photoelectricdetector, for relatively vibrating said photoelectric detector and saidimage of said alignment mark which is formed by said second objectiveoptical system, thereby detecting a position of said image of saidalignment mark; a third objective optical system for forming images ofsaid alignment mark and said reference mark from said second light beam;and an image pickup device for picking up said images of said alignmentmark and said reference mark, said images being formed by said thirdobjective optical system, wherein a positional relationship of said maskwith respect to said image position detector is detected on the basis ofa detection result from said image position detector, and a positionalshift between said alignment mark and said reference mark is detected onthe basis of a detection result from said image pickup device.
 2. Analignment apparatus according to claim 1, wherein said first objectiveoptical system is arranged to be movable in correspondence with theposition of said alignment mark on said mask.
 3. An alignment apparatusaccording to claim 1, further comprising a correction optical system,arranged between said wavelength selection optical system and saidsecond objective optical system, for converting the light beam from saidfirst objective optical system into a parallel beam, said correctionoptical system being movable in a manner interlocked with said firstobjective optical system.
 4. An alignment apparatus according to claim1, further comprising a correction optical system, arranged between saidwavelength selection optical system and said third objective opticalsystem, for converting the light beam from said first objective opticalsystem into a parallel beam, said correction optical system beingmovable in a manner interlocked with said first objective opticalsystem.
 5. An alignment apparatus according to claim 1, wherein saidsecond alignment illumination system performs incident-lightillumination of said alignment mark from an upper side of said mask. 6.An alignment apparatus according to claim 5, further comprising amovable mirror, arranged to be freely inserted between said mask andsaid projection optical system, for reflecting the continuousillumination light transmitted through said mask toward said alignmentmark.
 7. An alignment apparatus according to claim 6, further comprisinga field stop arranged on a plane conjugate with a plane where saidmovable mirror is arranged in said second alignment illumination system.8. An alignment apparatus according to claim 1, wherein said wavelengthselection optical system comprises a dichroic mirror.
 9. A projectionexposure apparatus comprising:an illumination optical system forilluminating a mask on which a target transferred pattern is formed witha pulsed exposure light; a projection optical system for projecting animage of said pattern on said mask onto a photosensitive substrate withthe exposure light; a substrate stage for supporting said substrate andpositioning said substrate on a plane perpendicular to an optical axisof said projection optical system; a first alignment illumination systemfor illuminating an alignment mark on said mask and a reference mark onsaid substrate stage with a pulse illumination light obtained uponbranching the pulsed exposure light; a second alignment illuminationsystem for illuminating said alignment mark with a continuously emittedcontinuous illumination light in a wavelength range different from thatof the exposure light; a first objective optical system for focusing thecontinuous illumination light from said alignment mark, the pulseillumination light from said alignment mark, and the pulse illuminationlight from said reference mark through said projection optical system; awavelength selection optical system for dividing a light beam focused bysaid first objective optical system into a first light beam of thecontinuous illumination light and a second light beam of the pulseillumination light; a second objective optical system for forming animage of said alignment mark from said first light beam; an imageposition detector, having photoelectric detector, for relativelyvibrating said photoelectric detector and said image of said alignmentmark which is formed by said second objective optical system, therebydetecting a position of said image of said alignment mark; a thirdobjective optical system for forming images of said alignment mark andsaid reference mark from said second light beam; and an image pickupdevice for picking up said images of said alignment mark and saidreference mark, said images being formed by said third objective opticalsystem, wherein a positional relationship of said mask with respect tosaid image position detector is detected on the basis of a detectionresult from said image position detector, and a positional shift betweensaid alignment mark and said reference mark is detected on the basis ofa detection result from said image pickup device.
 10. A projectionexposure apparatus according to claim 9, wherein said first alignmentillumination system comprising:a light guide for guiding a parallel beamfrom said illumination optical system to said alignment mark and saidreference mark; and a diffusion optical system, arranged between saidillumination optical system and said light guide, for diffusing theparallel beam, and wherein an incident end face of said light guide isarranged to be inclined by a predetermined angle with respect to a planeperpendicular to a direction of incidence of the parallel beam onto saiddiffusion optical system.